In a hydraulic motor vehicle brake system, the required brake fluid is located in the fluid reservoir, with the reservoir warning apparatus for monitoring the reservoir filling level usually having a reed switch with a reed contact as the switching element and usually having a magnet as the transducer.
As is known, a reed contact comprises two ferromagnetic reeds which are coated with noble metal and are fused into a glass body or small glass tube. The two reeds overlap to a minimum extent in this case. The small glass tube is evacuated before fusing and partly filled with nitrogen or a gas mixture with a high nitrogen content.
If the level of brake fluid falls and the magnet passes a predefined switching point, the reeds of the reed switch close on account of the generated magnetic field since the magnetic attraction force of the reeds exceeds the spring action of the individual reeds. The reed switch can, as described, be provided as a closer or NO switch in which the reeds are open in the inoperative position. However, it is also possible to use a reed switch which is in the form of an opener or an NC switch which has reeds which are closed in the inoperative state and are opened under the action of a magnetic force.
This signal is evaluated in a controller and the driver of the motor vehicle can be warned by way of an optical and/or acoustic indicator. In this case, it is necessary to ensure that the fluid reservoir is always filled with a statutorily predefined minimum amount of brake fluid in order to not compromise the functioning of the brake system. Reservoir warning apparatuses for monitoring the reservoir filling level in which a float, which is provided with the magnet, switches the reed contact as soon as the float assumes a position (switching point) in which the reservoir filling level falls below a defined minimum are generally used. A warning signal which can be sensed by the driver is triggered by the reed contact.
A fluid reservoir for a hydraulic motor vehicle brake system and a described switching unit are known, for example, from DE 10 2005 009 657 A1.
In line with current practice, the reservoir warning apparatuses are subject to a switching point test and a pivoting test (180° rotation) in the installed state, that is to say after installation of the reservoir warning apparatus into the fluid reservoir.
In spite of these tests, the switching units with reed contacts often fail since some manufacturer faults are not found by the switching point test and pivoting test and subsequent faults can occur in the field due to handling (transportation, assembly etc.).
Possible faults or failures which can occur in an installed reed contact in the field are:                Broken glass body (hairline cracks)        Leaks between the glass body and contacts        Floating particles in the glass body        Bent reeds in the glass body        Skewed orientation of the reeds in the glass body        Tendency for the reeds to stick in the contact region        